1. Field of the Invention
The present invention relates to systems and methods for transmitting data, and in particular to a system and method for measuring and correcting for audio to video synchronization errors.
2. Description of the Related Art
In presenting media programs such as films and television programs, it is important that the video portion of the program and the audio portion of the program be adequately synchronized. Timing differences between the audio and video portions of a media program can be caused by digital signal processing devices with unequal or separate audio and video time delays. Typically, video processing takes longer than audio processing, and the video signal is delayed from the audio signal. Such errors are most noticeable as lip synchronization (“lipsync”) errors when the media program displays someone speaking. Timing differences may occur in the production and distribution of programming during: initial production and mixing, editing and special effects, frame synchronization, compression for storage or transmission, and audio/video display or presentation. There are typically multiple problems and errors that may accumulate as a media program passes through different stages in the distribution chain. Added together, such errors can be objectionable to the viewer.
Many synchronization problems can be easily remedied by adjusting the delay of the audio signal or the video signal at a number of points in the distribution chain, if the error is known. Frame synchronizers, such as those disclosed in U.S. Pat. No. 4,218,705 are but one example of devices for making such adjustments. Still other equipment can perform out-of-service timing measurements, but when the equipment is placed in service to make in-service measurements during regular programming, timing errors are difficult to measure. In most cases, human operators must determine by observation if the synchronization between the audio and the video components of the media program is correct. It is difficult to determine the magnitude or direction (early or late) of lipsync error, and if a synchronization problem has been identified, it is difficult to determine which of the elements in the distribution chain is the source of the unacceptable error.
Many methods have been used to synchronize the audio and video of media programs. The earliest known method was the clapboard used in film productions, which allowed film producers to align the timing of the frame where the clapboard closed with the sound on the audio track. This sort of synchronization test is known as an out-of-service test. More sophisticated out-of-service equipment is in current use, including the VM700 test set available from the TEKTRONIX corporation, which measures timing errors between special test signals applied to the audio and video signal paths.
Other systems have attempted to address the synchronization issue. For example, TEKTRONIX also developed a system (the AVDC100) for in-service timing measurements. The AVDC 100 would make such measurements by carrying a replica of the audio signal in the active portion of the video signal in the form of a watermark. While effective, this approach was held proprietary and was not widely adopted, and the watermarking process lowers the signal to noise ratio (SNR) of the video and makes the video signal difficult to compress in MPEG systems, which causes a waste of precious bandwidth.
Other systems have been devised. For example, U.S. Pat. No. 6,836,295 discloses the use of audio events to mark video. However, this technique requires additional bandwidth or degradation of the video signal, and is not compatible across all television and film formats. This patent also refers to U.S. Pat. No. 4,703,355, and indicates that this patent discloses the addition of adding an audio signal for timing purposes. However, to assist with the problem of losing the timing signal, this patent discloses that the added signal is continuous, thus again requiring additional bandwidth and potentially compromising the original audio signal.
U.S. Pat. No. 5,243,424 discloses a system that detects “vision cuts” in a video signal, and after a known delay, adds a marking signal to the audio channel. However, this system relies on attenuation of the marking signal to below that of the audio portion of the media program, and this is not practical in many applications. This system is also incapable of precise synchronization, because no means is disclosed for determining the precise timing of the tone burst. U.S. Pat. No. 4,963,967 discloses a system that generates a video test field and a pulse every N number of frames, and applies that pulse to add a distinctive video signal and an audio tone. This system is more complex than necessary, and adds information to both the video and audio signals, wasting bandwidth and unnecessarily increasing complexity.
What is needed is a simple, yet effective method and system for synchronizing the video and audio portions of media programs. What is further needed is that this system be suitable for conversion between all television and film formats and be suitable for use with both audio and video compression techniques, while not requiring any additional bandwidth. The present invention satisfies that need.